This invention relates to printed circuit wiring boards, prepregs from which they are prepared and to copolymers from which the prepregs are prepared. The copolymers comprise repeating units of polynorbornene structures. Some of the structures have been substituted with a silane substitutent and some have not been substituted. The silane substitution provides increased adhesion to various substrates, for example, glass cloth and copper.
Printed circuit wiring boards generally comprise at least one layer of glass cloth which has been impregnated with a polymer or copolymer. These wiring boards are fabricated from prepregs. Prepregs are generally formed of glass cloth which has been impregnated with a polymer or copolymer and then partially cured so that further curing can take place. The prepregs are then generally arranged in a stacking sequence and subjected to heat and pressure so as to form a cured laminate.
The laminates generally comprise at least one layer of prepreg and at least one layer of a conductive film, such as copper, which when imaged and etched serve to provide a printed circuit wiring board substrate.
Typical printed circuit wiring boards are single layer or multilayer in configuration. The typical single layer printed circuit wiring board comprises a central prepreg/substrate layer to which have been laminated two layers of conductive film (optionally only a single layer of conductive film may be laminated to the substrate layer). The conductive film is then imaged and etched to form the printed circuit mentioned above. If a multilayer printed circuit wiring board is desired, this procedure can be repeated. Thus, typically, a structure similar to a single layer printed circuit wiring board is fabricated comprising a prepreg substrate core and imaged and etched conductive film patterns on each major surface of the substrate. Subsequently, additional layers of prepreg are configured in a stacking sequence on each major surface together with one or two layers of conductive film on the exterior major surfaces of the additional prepreg layers. The sequence is then subjected to heat and pressure in order to form a multilayer laminate comprising layers of conductive film, prepreg, conductive film, prepreg, etc..
Cellulosic and fiberglass cloths have long been used to reinforce polymeric substrates such as the prepregs discussed above. It is known that silane coupling agents can be applied directly to glass filaments to improve the properties, such as the strength of laminates such as those discussed above, often by as much as 300% for compression molded test samples. It is believed that silane coupling agents at the interface allow many particulate materials to act as reinforcing fillers in such laminates to increase various properties, such as strength, hardness, modulus, heat distortion and impact strength. Fiberglass cloth is usually treated with a solution of a coupling agent. This coupling agent can be a silane coupling agent and can be applied directly to the glass cloth.
The precise nature of the mechanism in which silane increases adhesion is not entirely understood. Silane coupling agents modify the interface between organic resin surfaces and non-resins to improve adhesion. The physical properties are improved as a result. It is possible that the silane coupling agents form bonds with the non-resin surfaces and resin surfaces through the silane functional group. Hydrolyzed silanes condense to oligomeric siloxanols and eventually to rigid cross-linked structures. Contact with a polymer matrix should take place while the siloxanols still have some solubility. The bonding to a polymer matrix may take various different forms. Bonding may be covalent where the siloxanol is compatible with the liquid matrix resin. The solutions might also form an interpenetrating polymer network as the siloxanols and the resins separately cure with only limited copolymerization.
It is well known that not all silanes or mixtures of silanes will bond all metals to all substrates. In McGee, U.S. Pat. No. 4,315,970, it is stated that:
[i]t is generally accepted that specific silanes can be used for adhesion of specific materials to specific substrates. That is, the silane must be matched to the application and it cannot be assumed that all silanes will work in all applications.
This statement illustrates the unpredictability of the suitability of silane coupling agents in improving adhesion of a metal to a substrate. Thus, this suitability must be determined by experimentation.
While suitable coupling agents are commercially available for bonding of many common plastics with a variety of metals, the application of silane coupling agents for bonding of polynorbornenes to metals has only recently been developed (see U.S. Ser. No. 228,034, filed Aug. 4, 1988 and commonly assigned to the same assignee as the present invention). Norbornene-type monomers are polymerized by either a ring-opening mechanism or by an addition reaction wherein the cyclic ring structure remains intact. Ring-opening polymerization generally yields unsaturated linear polymers while addition polymerization yields polycycloaliphatics. It is desirable to produce polymers having high molecular weight repeating units incorporated therein to provide good temperature resistance, i.e., high heat distortion temperatures and high glass transition temperatures.
CA98:162025n and 98:162026p disclose laminates for use in preparing printed circuit boards. The laminates comprise an assembly of prepregs of paper-reinforced phenolic resin and copper foil. Polyethylene may be employed as an intermediate layer between the copper foil and the prepregs. The polyethylene layer is silane-modified in the presence of a radical-generating agent and is employed as an adhesive layer.
CA107:8574p discloses laminates of glass fibers impregnated with silicon-modified epoxy resins which also contain polyethylene. A six-layered wiring board is prepared from 15 sheets of the prepreg and 6 sheets of copper foil. CA107:8575q discloses similar laminates wherein epoxy resins, guanidine derivatives, fluoro-plastics or polyolefins are employed as the resin.
"Some Approaches to Low-dielectric Constant Matrix Resins for Printed Circuit Boards", Butler, et al., 15th National SAMPE Technical Conference, 1983, discloses general design considerations in the preparation of printed circuit boards. It discloses that the thermal cyclization of materials to form multicyclic structures has been employed in the preparation of printed circuit boards. It discloses that "conventional silane reactions" may be employed to overcome the shortcomings of silicon and that siloxane is a "desirable group[s] for polymer segment structures". It also discloses that coupling agents to improve adhesion can be employed.
Although printed circuit wiring boards employing various materials, e.g., those discussed above, are available, serious deficiencies in properties still exist, such as good adhesion, low dielectric constant, good punchability, good molten solder resistance characteristics and improved peel strength. Prior art printed circuit wiring board substrates fall short of optimizing these parameters and providing a spectrum of properties which is optimal. Thus, there has been a continuing need for improvement.